A 'waifu' is a joking moniker applied to characters that we feel to be really neat or adorkable or adorable or what have you, to which one can become through some aspect of their design, personality, etc effectively become attached/really genuinely enjoy as a character.

The male moniker thereof is known as 'Husbando'

So for example I really enjoy the character of Sucy from LWA, really unique smart alecky girl with a penchant for magical poisons and stuff. She's one of my 'waifus'

I also really like the 10th Doctor he's pretty cool, doesn't afraid of anything and he's the first Doctor I ever genuinely enjoyed watching and what got me hooked on the show, effectively he's a 'husbando'

Silly jokey crap that doesn't mean anything beyond the fact that we like certain characters and are all weaboos.

I just remembered that my first waifu was Ninjara.I saw her in my teenage mutant ninja turtles comic book as a kid and I was lost in her amazingness and beauty.I remember laying in bed as a small kid and admiring her well drawn boobs and I remember hating Raphael real bad because he was dating her...Hehehe, I even felt som of that old jealousy surface when I googled pictures of her for this post.